Method of making electric lamps



NOV. 23, 1965 DAl ls METHOD OF MAKING ELECTRIC LAMPS Filed Feb. 5, 1962 ATTORNEYS United States Patent Ofilice 3,2l9A93 Patented Nov. 23, 1965 3,219,493 NIETHOD OF MAKING ELECTRIC LAMPS Jeanne A. Dainis, Somerville, Mass., assignor to Polaroid Corporation, Cambridge, hdass a corporation of Delaware Filed Feb. 5, 1962, Ser. No. 170,926 8 Claims. (Cl. 148-13) This invention relates to incandescent lamps adapted to be operated at relatively high filament temperatures and more particularly to the formation of tantalum carbide filaments for use in such lamps.

A principal object of the present invention is to provide a method of producing filaments comprising a major percentage of tantalum carbide.

Another object of the invention is to provide a method of converting shaped tantalum filaments to tantalum carbide without undesirable distortion in the previously established structure.

A further object of the present invention is to provide a method of producing tantalum carbide filaments which does not require a carbonaceous atmosphere.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the method encompassing the several steps and the relation of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of this invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:

FIGURE 1 is a representation of a section through a typical vehicle headlamp of the sealed-beam type wherein a tantalum carbide filament such as produced by the process of the invention is used;

FIGS. 2 and 3 are enlarged views of a preferred type of filament used in the lamp of FIG. 1; and

FIG. 4 is a view of the filament of FIG. 3 enclosed in a carburizing envelope as set out in the invention.

There is a demand for incandescent lamps adapted to be operated at relatively high filament temperatures and possessing relatively long useful operating life as well as, for example, high emissivity. Such lamps would be desirable, for example, in vehicle headlighting, flood lighting, picture projection and the like. To achieve such incandescent lamps, it has been heretofore proposed to employ a filament comprising essentially tantalum carbide such as disclosed in US. Patent 2,596,469 issued May 13, 1952, in the name of Dexter P. Cooper, Jr., or they may comprise a solid solution or mixture of a major percentage of tantalum carbide and a minor percentage of at least one other refractory metal carbide such as disclosed in copending US. application Serial Nos. 5,524 and 5,525 both filed January 29, 1960 (now US. Patents Nos. 3,- 022,436 and 3,022,437, both issued February 20, 1962). In the former application, there is disclosed and claimed the use of a carbide filament comprising tantalum carbide and from 1 to about 30 percent by weight of at least one metal carbide selected from the group consisting of the carbides of zirconium and hafnium. In the latter-mentioned application, there is disclosed and claimed the use of a carbide filament comprising tantalum carbide and between 1 and 10 percent by weight of at least one refractory metal carbide selected from the group consisting of the carbides of titanium, thorium, vanadium, niobium, molybdenum, tungsten and uranium.

One convenient method for forming a preferred carbide filament comprises converting a tantalum or suitable tantalum alloy filament to the carbide structure after it has been shaped into the desired configuration or geometry. This conversion may be achieved by passing sufficient current through a filament in a carburizing atmosphere until the desired carbide structure is obtained. Carburizing techniques such as described above are more fully set forth, for example, in US. Patent 2,596,469. Other carburizing methods uch as, for example, zone carburization may also be employed to carry out the conversion.

In many applications, there is necessarily utilized at least one filament having a geometry, e.g., a coil or a coiled-coil configuration, which enhances or provides for the obtaining of the greatest filament brightness or brilliance. Thus, after the formation of such filaments, it is essential that the geometry thereof be maintained during subsequent handling so that the desired filament brightness may be obtained. Frequently, however, during the production of tantalum carbide filaments having a coiled or coiled-coil structure, undesirable alteration of the coil geometry results.

Another method of producing such filaments, as disclosed in the copending US. application of Dexter P. Cooper, Jr., entitled Electric Lamps, Serial No. 171,- 292, filed of even date herewith, uses an organic polymeric mandrel about which the tantalum filament is coiled for shaping the desired configuration. This mandrel is then either dissolved prior to or burned off during the carburizing process of converting the tantalum to tantalum carbide.

The present invention contemplates the use of a carbonaceous material to coat the filament. The filament is then carburized within the envelope formed by the coating. A preferred carbonaceous material is obtained by using a viscous liquid, in combination with carbon added to the liquid, which fixes the structural configuration of the filament when applied as a coating on the filament. This coating is removed subsequent to carburization of the filament. By combining carbon with the liquid, a captive carburizing method is provided, surrounding all parts of the filament, which accomplishes conversion of the filament to the carbide thereof without complete reliance on an external carburizing atmosphere which prior techniques have required.

Referring now to the drawings, FIG. 1 illustrates a sealed-beam vehicle headlamp which comprises a cupshaped base member 10 having its inner surface 12 silvered, aluminized or otherwise coated to provide a reflecting surface. The base member 10 has hermetically sealed thereto a transparent cover plate 14, of, for example, glass, which may serve as a lens element for controlling the dispersion of light emitted from the lamp. Within the bulb there is mounted or suspended a preferred compound coil carbide filament 16 on lead wires 13, which, in turn, are attached to subleads 20. The subleads may be connected to a source of electric power outside the bulb or envelope. The filament is located at the focus of the reflector. After assembly, the bulb is evacuated and filled with a regenerative atmosphere 22 which, for example, may comprise a source of carbon, a source of hydrogen and a source of at least one halogen, e.g., chlorine, and, if desired, an inert gas of low heat conductivity such as argon. The compounds of the regenerative atmosphere may be provided by introducing one or more suitable materials before the bulb is sealed.

While FIG. 1 particularly describes a vehicle headlamp, it is to be understood that the filaments as produced by the present invention are applicable to incandescent lamps generally, for example, photoflood lamps and other related structures. Furthermore, while FIG. 1 describes a specific lamp configuration or structure,

it is understood that the incandescent lamp may take any desired shape and have any desired size.

Referring now to FIGS. 2 and 3, there is illustrated a preferred filament which may be employed in the incandescent lamps described above. In FIG. 2, there is shown a wire filament 30 comprising a major percentage of tantalum in uncoiled form. The filament 30 comprises a core wire 32 of suitable diameter and a fine wire 34 comprising a major percentage of tantalum of a diameter smaller than the core wire 32 wound about the core wire. The fine wire 34 comprises a helix extending along the length of core wire 32. For high brightness, the pitch of the fine wire is carefully con trolled during winding. The resulting composite or compound wire structure is then coiled to produce a filament as illustrated in FIG. 3. This type of filament structure is more fully disclosed in the copending US. application of George R. Bird, entitled Electric Lamps, Serial No. 170,907, filed of even date herewith.

To effect a filament having the high brightness and other advantages of the coiled-coil or compound wire structure described above, it is necessary that the filament retain its shape during carburization. The method of manufacture of tantalum carbide filaments outlined in the present invention not only provides such a shape-retaining envelope, but also permits conversion in a non-carburizing as well as a carburizing atmosphere.

In the process of the present invention, a formed tantalum filament is coated with a viscous air hardenable liquid, as for example, a thermosetting resin, such as an air-dry varnish or preferably melamine formaldehyde, and then covered with carbon by spraying thereon powered graphite or dipping the coated coil in powdered graphite. The process of coating the filament with a complete carbon envelope may also be accomplished by combining a large percentage of carbon with the viscous liquid and then coating the filament with the resulting mixture. The first method is preferred as the combination of the liquid and carbon prior to coating tends to produce a rather thick mixture which is more difiicult to apply to the filament. However, both methods produce the desired end result.

FIG. 4 illustrates one form of a filament, in this case a coiled-coil or compound wire structure 30 consisting of a smaller diameter wire 34 wound in a predetermined helix pattern about a larger diameter wire 32, which has been coated with a carbon envelope 36 comprising a viscous liquid and carbon either in combination or with the carbon applied subsequent to the application of the liquid. By reason of the viscous properties of such a mixture, it is possible to cover all surfaces of the compound wire structure with the said carbon envelope to ensure complete carburization and transformation of the tantalum filament to the carbide thereof.

Although the illustration of FIG. 4 shows a coiled-coil structured filament, the process of the present invention is particularly applicable to those filaments wherein it is desired to retain a complex initial shape by fixing the design thereof during carburization.

The coated filament is then inserted in an atmosphere for the carburization of filament. This atmosphere may be any atmosphere. Preferred atmospheres are of a carbonaceous type comprising a volatile hydrocarbon and hydrogen, wherein the volatile hydrocarbon may be ethylene or methane as preferred compounds. A major advantage of the present method, however, is that carburization can also be accomplished in a non-carbonaceous atmosphere, as for example, an inert gas such as argon, xenon, or krypton.

After insertion in an atmosphere as described above, the filament is therein operated at an elevated temperature by means of passing current through the filament or by a combination of heat applied to the coated filament internally, as described above, and externally, by known means such as an oven. The operation of the filament at an elevated temperature carburizes the filament within the coating envelope and then burns off the coating envelope leaving the tantalum filament converted to the carbide thereof. In practice, the filament is initially operated for a period of from five to thirty minutes at a temperature below 2,000 K. During this period, a charring action occurs within the coating envelope. This action begins at approximately 1,000 K. at which point drying of the liquid carbonaceous material, such as a thermosetting resin, is apparent.

Subsequent to the period of charring, the filament temperature is gradually elevated to substantially 3,200 K. fora period of approximately ten minutes to burn olf the carbonaceous material envelope shell. This gradual elevation may be accomplished within a period of approximately five minutes. If desired, as the temperature of the filament is elevated, a series of step temperature increments may be made, holding the temperature at a particular level until all of the carbon which will come oif at that temperature is removed and then proceeding to the next level at which additional carbon will burn off of the filament.

The resultant tantalum carbide filament may then be removed from the precarburization atmosphere in which it has been processed for insertion in a final operating lamp of the desired configuration and properties.

The complete carbonaceous material envelope of the filament structure as taught in this invention seals in the tantalum filament to prevent cracks and holes from forming in the tantalum during operation at elevated temperatures at which conversion to tantalum carbide occurs. As an additional safeguard, the atmosphere in which carburization takes place may be of a carbonaceous nature or an inert gas as described in detail above. The carbonaceous material envelope of the process thus acts as a carburizer and as a mechanical lock to fix the design of the filament structure.

Since certain changes may be made in the above method without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying draw ing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising coating a shaped filament containing a major percentage of tantalum with a viscous, air hardenable liquid which adheres thereto and hardens thereon to form a self-supporting envelope, and carburizing the said coated filament through said envelope whereby the shape of said filament is retained by said envelope during carburization.

2. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous air hardenable material about a shaped filament containing a major percentage of tantalum, which said coating adheres thereto and hardens thereon to form a self-supporting envelope, and carburizing said filament whereby the shape of said filament is retained by said envelope during carburization.

3. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous air hardenable material about a shaped filament containing a major percentage of tantalum, said carbonaceous material being a thermosetting resin, which said coating adheres thereto and hardens thereon to form a selfsupporting envelope, and car-burizing said filament where- :by the shape of said filament is retained by said envelope during carburization.

4. The method of claim 3 wherein the said thermosetting resin is melamine formaldehyde.

5, In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous viscous, air hardenable liquid about a shaped filament containing a major percentage of tantalum,

which said coating adheres thereto and hardens thereon to form a self-supporting envelope, inserting said coated filament in a carbonaceous atmosphere, and heating said filament within said atmosphere at an elevated temperature whereby said filament is converted to a major percentage of tantalum carbide and the shape of said filament is retained by said envelope during said heating.

5. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous viscous, air hardenable liquid about a shaped filament containing a major percentage of tantalum, which said coating adheres thereto and hardens thereon to form an envelope, inserting said filament in a self-supporting inert gas atmosphere, and heating said filament within said atmosphere at an elevated temperature whereby said filament is converted to a major percentage of tantalum carbide and the shape of said filament is retained by said envelope during said heating.

7. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous viscous, air hardenable liquid about a shaped filament containing a major percentage of tantalum, which said coating adheres thereto and hardens thereon to form a self-supporting envelope, inserting said coated filament in a carbonaceous atmosphere, heating said filament within said atmosphere to a filament temperature less than 2000 K. for a predetermined period and gradually increasing said filament temperature to substantially 3200 K. and maintaining said last-named temperature for a predetermined period whereby the said filament is converted to the carbide thereof and the shape of said filament is retained by said envelope during said heating.

8. In a process of manufacturing tantalum carbide lamp filaments, the steps comprising forming a coating with a carbonaceous viscous, air hardenable liquid about a filament containing a major percentage of tantalum, which said coating adheres thereto and hardens thereon to form a self-supporting envelope, inserting said coated filament in a carbonaceous atmosphere, heating said filament within said atmosphere to 'a filament temperature greater than 1000 K. and less than 2000 K. for a period of time greater than five minutes and less than thirty minutes, and gradually increasing said filament temperature to substantially 3200 K. and maintaining said last-named temperature for a period of the order of ten minutes whereby said filament is converted to the carbide thereof and the shape of said filament is retained by said envelope during said heating.

References Cited by the Examiner UNITED STATES PATENTS 1,209,247 12/ 1916 Bastian 292.5.l4 2,218,345 10/ 1940 Spaeth 2925.18 2,619,706 12/ 1952 Uause 29-25.14 3,113,893 '12/ 1963 Sloan 14813 FOREIGN PATENTS 450, 114 7/ 1948 Canada.

BENJAMIN HENKIN, Primary Examiner.

LEON PEAR, DAVID L. RECK, Examiners. 

1. IN A PROCESS OF MANUFACTURING TANTALUM CARBIDE LAMP FILAMENTS, THE STEPS COMPRISING COATING A SHAPED FILAMENT CONTAINING A MAJOR PERCENTAGE OF TANTALUM WITH A VISCOUS, AIR HARDENABLE LIQUID WHICH ADHERES THERETO AND HARDENS THEREON TO FORM A SELF-SUPPORTING ENVELOPE, AND CARBURIZING THE SAID COATED FILAMENT THROUGH SAID ENVELOPE WHEREBY THE SHAPE OF SAID FILAMENT IS RETAINED BY SAID ENVELOPE DURING CARBURIZATION. 